1. Field of Invention
The present invention relates to a thin film forming method including arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent. The present invention also relates to a thin film forming method including ejecting the solution to arrange a plurality of droplets of the solution on a substrate, and evaporating a solvent from each droplet, thereby forming a thin film on the substrate.
2. Description of Related Art
Recently, attention has been focused on electronic devices employing an organic thin film (thin film made of an organic substance) as a functional thin film. An example one of such electronic device is an organic EL device. One example of an organic thin film for use as a light emitting layer in an organic EL device is a thin film of Alq3 (quinolinol-aluminium complex) formed by vacuum deposition, for example. When that thin film is formed by an ordinary vacuum deposition process, it is not obtained in a crystal state, but in an amorphous state.
In the related art, a crystalline thin film of Alq3 can be obtained with the vacuum deposition process by providing a layer of fulleren as an underlying layer (see, e.g., Japanese Unexamined Patent Application Publication No. 10-41070). This Publication also discloses that, by employing, as a light emitting source, a crystalline Alq3 thin film formed by the disclosed method, the light emission efficiency of an organic EL device can be enhance from that in the case employing, as a light emitting source, a crystalline Alq3 thin film formed by the ordinary vacuum deposition process.
Also, in the related art, a crystalline organic thin film can be formed using a liquid phase process. Depending on the kinds of materials, a crystalline organic thin film can be formed, for example, by a method of spin-coating a solution of an organic substance. Examples of available materials includes a-sex-ithiophene, hexadecafluorocopper phthalocyanine, and naphthalene tetracarboxyl diimide.
On the other hand, a functional thin film is patterned when used in many electronic devices. However, it is difficult to pattern a crystalline organic thin film by an ordinary patterning process including photolithography and etching because of low resist resistance of the organic substance. The related art regarding various crystalline organic thin films of the above-mentioned type do not pattern the crystalline organic thin films. Accordingly, a selectable range of functional thin film materials for practical use is restricted and any desired material cannot always be employed. Furthermore, although crystallinity directly affects physical properties, the related art does not provide details regarding that effect.
If it is possible to arrange a very small amount of solution of an organic substance in a predetermined position on a substrate by an ink jet method and to crystallize the arranged solution, a patterned crystalline organic thin film can be easily formed on the substrate. Also, this method is able to form a crystalline thin film from all kinds of materials that can be dissolved into a solution, and to produce a perfect crystal (single crystal) in principle.
A thin film formed in the related art by the ink jet method is made of a high-molecular compound. Such a thin amorphous film made of a high-molecular compound can be readily formed by dissolving the high-molecular compound in a solvent to obtain a solution, arranging the solution on a substrate by the ink jet method, and evaporating the solvent from the arranged solution.
However, when making an effort to form a thin film using a compound having a relatively low molecular weight and not classified into high-molecular compounds (hereinafter xe2x80x9clow-molecular compoundxe2x80x9d) by the ink jet method similarly to the above-mentioned case using a high-molecular compound, a thin film made of the low-molecular compound is not formed and particles of the low-molecular compound are deposited on the substrate.
Such a result is attributable to the fact that, even with the substrate treated to become lyophilic, the cohesion among molecules of the low-molecular compound is much greater than the binding force between the low-molecular compound and the substrate. Thus, because a low-molecular compound has a great cohesion, it is a material having high crystallinity.
Further, when ejecting a solution by the ink jet method and arranging droplets on a substrate, if a partial pressure of a gas made up of the same components as those of a solvent for the solution forming the droplets is not uniform around each droplet, the droplet is distorted and tends to easily move toward the side in which the partial pressure is higher. That movement of the droplets is apt to particularly occur when a solution of a low-molecular compound having a weak binding force with respect to the substrate is ejected. Then, the movement of the droplets raises a difficulty in forming a thin film in a predetermined position on the substrate. As a result, the ununiformity of the partial pressure of the gas prevents the thin film from crystallized.
For example, when a plurality of droplets are formed along a line at small intervals between the droplets, the partial pressure around the droplet formed at the most end of the line is not uniform because the gas partial pressure is lower on the side where the adjacent droplet is not present and higher on the side where the adjacent droplet is present. The-reason that this occurs is because, around the droplet formed at the most end of the line, the gas partial pressure is higher on the side, in which the adjacent droplet is present, with the presence of solvent vapors vaporized from the adjacent droplet.
Accordingly, the present invention provides a method capable of crystallizing a very small amount of solution arranged in a predetermined position on a substrate so that a patterned crystalline thin film (particularly an organic thin film) can be easily formed on the substrate by the ink jet method.
To address or achieve the above, the present invention provides a thin film forming method including: arranging, on a substrate, a plurality of droplets of a solution prepared by dissolving a thin film forming material in a solvent, and evaporating the solvent from each droplet, thereby forming a thin film on the substrate. The gas partial pressure around each droplet formed on the substrate is kept uniform to prevent the above-mentioned movement of the droplet so that a thin film of a low-molecular compound can be formed in a predetermined position on the substrate.
Thus, to address or achieve the above, the present invention provides a thin film forming method including: arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent, controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of the droplets, thereby creating crystalline nuclei in the droplets, and then growing the crystalline nuclei to form a crystalline thin film.
Also, as a thin film forming method for practically implementing a method of controlling the partial pressure, the present invention provides a thin film forming method including: arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent, creating crystalline nuclei in the droplets by controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of each of the arranged droplets to a first partial pressure (e.g., a partial pressure equal to or substantially equal to the saturation vapor pressure) under which the solution forming the droplets is brought into a supersaturated state, and after creation of the crystalline nuclei, lowering the partial pressure of the gas in the vicinity of the droplets to a second partial pressure (e.g., {fraction (1/10)} to {fraction (1/100)} of the saturation vapor pressure) under which the crystalline nuclei are able to grow into crystals.
Further, as another thin film forming method for practically implementing a method of controlling the partial pressure, the present invention provides a thin film forming method including: arranging, on a substrate, droplets of a solution prepared by dissolving a thin film forming material in a solvent, creating crystalline nuclei in the droplets by bringing the solution forming the arranged droplets into a supersaturated state and by controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of the droplets to a first partial pressure (e.g., a partial pressure equal to or substantially equal to the saturation vapor pressure) under which the solvent is hard to evaporate from the solution forming the droplets, and after creation of the crystalline nuclei, lowering the partial pressure of the gas in the vicinity of the droplets to a second partial pressure (e.g., {fraction (1/10)} to {fraction (1/100)} of the saturation vapor pressure) under which growth of the crystalline nuclei into crystals occurs with priority to creation of further crystalline nuclei.
With the last-mentioned method, the solution forming the droplets immediately after being arranged on the substrate is first brought into a supersaturated state, whereby crystalline nuclei necessary for crystallization are created in the solution. Then, the partial pressure of the gas (i.e., the gas made up of the same components as those of the solvent) in the vicinity of the droplets is lowered from the first partial pressure (i.e., a high partial pressure under which the solvent is hard to evaporate from the solution forming the droplets) to the second partial pressure (i.e., a low partial pressure under which the solvent is easy to evaporate from the solution forming the droplets and which growth of the created crystalline nuclei into crystals occurs with priority to creation of further crystalline nuclei).
Accordingly, with that method, a patterned crystalline thin film can be easily formed on the substrate, for example, by arranging the droplets in a predetermined pattern by the ink jet method.
In the case in which the volume of each of the droplets arranged on the substrate is very small on the order of, e.g., 20 picoliter as resulted when employing, for example, the ink jet method in the droplet arranging step, if the partial pressure of the gas (i.e., the gas made up of the same components as those of the solvent of the solution forming the droplets) in the vicinity of the droplets is low, the solvent is apt to easily evaporate from the droplets and the concentration of the solution forming the droplets is quickly increased. Hence, a degree of supersaturation of the solution is also quickly increased, whereby a number of crystalline nuclei are formed and a solute tends to be powdery. In contrast, according to the method of the present invention, since the partial pressure of the gas in the vicinity of the droplets immediately after being arranged is controlled to the first partial pressure (i.e., a high partial pressure under which the solvent is hard to evaporate from the solution forming the droplets), the concentration of the solution forming the droplets is made uniform and stabilized in a supersaturated state in which a degree of supersaturation is relatively low (namely, an increase rate of degree of supersaturation of the solution forming the droplets is moderated). As a result, a small number of nuclei (ideally one) is created.
Further, to form a single-crystal thin film, it is required that after one nucleus has been created, only the one nucleus is grown into a crystal while reducing or preventing creation of other nucleus. If the partial pressure of the gas in the vicinity of the droplets immediately after being arranged remains high, further nuclei are created. In contrast, according to the method of the present invention, after the creation of the crystalline nuclei, the partial pressure is reduced to the lower partial pressure under which growth of the created crystalline nuclei into crystals occurs with priority to creation of further crystalline nuclei (i.e., the second partial pressure), whereby the crystal growth is accelerated while reducing or preventing the creation of further crystalline nuclei.
According to the method of the present invention, therefore, the degree of supersaturation of the solution forming the droplets can be quickly increased and a crystalline thin film of a single crystal can be obtained by quickly lowering the partial pressure from the first partial pressure to the second partial pressure immediately after a small number of crystalline nuclei (ideally one) have been created in the solution, for example, by lowering the partial pressure from the first partial pressure, which is equal to or substantially equal to the saturation vapor pressure, to the second partial pressure of 1.3 Pa (10-2 torr) in a time of 1 to 10 seconds.
In the method of the present invention, a partial pressure control method for controlling the partial pressure to the first partial pressure can be implemented, for example, by (1) adjusting an ejection interval (array pitch) of the droplets, (2) adjusting an amount of the ejected solution (i.e., an amount of the solution forming each of the droplets), or (3) adjusting the partial pressure of the gas in positions where the droplets are to be arranged, prior to the droplet arranging step.
In the method of the present invention, a method for relatively lowering the partial pressure from the first partial pressure to the second partial pressure can be implemented, for example, by (1) depressurizing an atmosphere in the vicinity of the droplets, (2) raising temperature in the vicinity of the droplets (this method (2) is intended to relatively lower the partial pressure by changing the saturated vapor pressure instead of changing an absolute value of the partial pressure), or (3) replacing an atmosphere in the vicinity of the droplets with an atmosphere of inert gas.
In the method of the present invention, the solution forming the droplets immediately after being arranged must be brought into a supersaturated state. To that end, it is preferable to employ one of (1) a solution containing the thin film forming material in amount at which the solution is brought into the saturated state when the solution is ejected in the droplet arranging step, (2) a solution containing the thin film forming material in amount at which concentration of the solution becomes not less than 1/10 of the saturated concentration but less than the saturated concentration at the time of ejection of the solution, and (3) a solution containing the thin film forming material in amount at which the solution is brought into the supersaturated state at the time of ejection of the solution.
In the method of the present invention, as stated above, the solution forming the droplets immediately after being arranged on the substrate is brought into the supersaturated state at a relatively low level to create a small number of crystalline nuclei, and after the creation of the crystalline nuclei, the degree of supersaturation of the solution forming the droplets is quickly increased for acceleration of the crystal growth. To that end, the solution used in the method of the present invention is advantageously one that can be brought into the supersaturated state over a wide concentration range.
Herein, the term xe2x80x9cdegree of supersaturationxe2x80x9d is defined by (Cxe2x80x94Cs)/Cs where C is the concentration and Cs is the saturated concentration. Also, the term xe2x80x9cpoor solventxe2x80x9d means a solution having a small saturated concentration. By employing a poor solvent as the solvent, therefore, a solution capable of being brought into the supersaturated state over a wide concentration range is obtained. Also, the use of a poor solvent is advantageous in that the interaction between solutes is relatively small even in the supersaturated state, and hence flexibility required in forming crystals can be obtained at a sufficient level.
Thus, according to the method of the present invention, by employing, as the solvent, a poor solvent (e.g., a solvent having such a characteristic that the saturated concentration of the thin film forming material at the time of ejection of the solution is not less than 0.1 weight % but not more than 10.0 weight %, or a solvent having such a characteristic that the saturated concentration of the thin film forming material at the time of ejection of the solution is not less than 0.1 weight % but not more than 1.0 weight %), it is possible to easily achieve creation of a small number of crystalline nuclei and acceleration of the crystal growth. On the contrary, in the case employing a good solvent to prepare a solution that is brought into the supersaturated state, if a solute is, e.g., a highly crystalline organic substance, a solution having high viscosity and low fluidity is resulted and hence crystalline nuclei are hard to grow.
In the method of the present invention, the solution used is preferably one containing the thin film forming material in an amount at which the concentration of the solution is not less than {fraction (1/10)} (including the saturated concentration and the supersaturated state) at the time of ejection of the solution. With this feature, the solution forming the droplets arranged on the substrate is more easily brought into the supersaturated state immediately after being ejected by the ink jet method, for example.
In the method of the present invention, dodecylbenzene is preferably used as the solvent. Dodecylbenzene becomes a poor solvent when an organic substance having high crystallinity, such as oligothiophene, is a solute. Also, since dodecylbenzene has a low vapor pressure, the use of dodecylbenzene enables the droplets to be easily controlled the supersaturated state.
In the method of the present invention, 2,3-dihydrobenzofuran is preferably used as the solvent. 2-3-Dihydrobenzofuran becomes a poor solvent when an organic substance having high crystallinity, such as oligophenylene, is a solute. Also, since 2-3-dihydrobenzofuran has a low vapor pressure, the use of 2-3-dihydrobenzofuran enables the droplets to be easily controlled the supersaturated state.
In the method of the present invention, dimethylformamide is preferably used as the solvent. Dimethylformamide becomes a poor solvent when an organic substance having high crystallinity, such as derivatives of oligophenylene or oligothiophene, is a solute. Also, since dimethylformamide has a low vapor pressure, the use of dimethylformamide enables the droplets to be easily controlled the supersaturated state.
The thin film forming material usable in the method of the present invention is, for example, oligophenylene or a derivative thereof, or oligothiophene or a derivative thereof. Oligophenylene is represented by the following chemical formula (1), and oligothiophene is represented by the following chemical formula (2). In each formula, n is equal to 2 or more. Also, in any case, n is preferably not less than 2 but not more than 6. 
One example of oligophenylene is p-terphenyl represented by the following chemical formula (3). One example of oligothiophene is 2,2xe2x80x2:5xe2x80x2,2xe2x80x3-terthiophene represented by the following chemical formula (4). One example of oligophenylene derivatives is 4-amino-p-terphenyl represented by the following formula (5). One example of oligothiophene derivatives is 2,2xe2x80x2:5xe2x80x2,2xe2x80x3-terthiophene-5,5xe2x80x3-dicarboxyaldehyde represented by the following formula (6). 
Another example of the thin film forming material usable in the method of the present invention is Alq3 (quinolinol-aluminium complex) represented by the following chemical formula (7) 
The solution used in the method of the present invention is, for example, one selected from among a solution in which the thin film forming material is oligophenylene and the solvent is 2,3-dihydrobenzofuran, a solution in which the thin film forming material is oligothiophene and the solvent is dodecylbenzene, and a solution in which the thin film forming material is a derivative of oligophenylene or a derivative of oligothiophene and the solvent is dimethylformamide.
Meanwhile, when a compound having a relatively low molecular weight and not classified into high-molecular compounds (hereinafter a xe2x80x9clow-molecular compoundxe2x80x9d) is ejected onto a substrate and then naturally dried, a thin film made of the low-molecular compound is not formed and particles of the low-molecular compound are deposited on the substrate. Such a result is attributable to the fact that, even with the substrate treated to become lyophilic, the cohesion among molecules of the low-molecular compound is much greater than the binding force between the low-molecular compound and the substrate. Because a low-molecular compound has a great cohesion, it is a material having high crystallinity. Thus, the thin film forming material used in the method of the present invention is a low-molecular compound.
According to the method of the present invention, by ejecting the solution after treating the substrate into a surface condition having an affinity with the thin film forming material (feature a), the thin film forming material dissolved in the ejected solution remains on the substrate in the surface condition having an affinity with the thin film forming material. As a result, the binding force between the thin film forming material made of the low-molecular compound and the substrate becomes higher than the cohesion among molecules of the thin film forming material, and the thin film forming material is stably cohered on the substrate, whereby a crystalline thin film can be more easily formed with stability.
As a method of treating the substrate into the surface condition having an affinity with the thin film forming material, it is preferable to employ a method of forming a self-assembly film on the substrate.
Herein, the term xe2x80x9cself-assembly filmxe2x80x9d means a dense single-molecule film in which a compound having a functional group capable of binding with a constituent atom of the film forming surface is made present in a gaseous or liquid state together with the film forming surface, whereby the functional group is adsorbed onto the film forming surface and is bound to the constituent atom of the film forming surface so that normal chain molecules are formed toward the outer side. Such a single molecule film is called a self-assembly film because it is formed based on spontaneous chemical adsorption of a compound onto the film forming surface.
The self-assembly film is explained in detail in Chapter 3, xe2x80x9cAn Introduction to Ultrathin Organic Film from Langmuir-Blodgett to Self-Assemblyxe2x80x9d, by A. Ulman (Academic Press Inc. Boston, 1991).
That method of adjusting the surface condition of the substrate with formation of the self-assembly film is a method that is capable of ensuring electrical continuity between the substrate and the thin film when the thin film formed by the method of the present invention is used as a functional thin film in an electrical device.
To obtain the substrate surface condition having an affinity with the thin film forming material by forming the self-assembly film, it is preferable, for example, to form, on the substrate, a self-assembly film made up of molecules each having an atomic group common to molecules constituting the thin film forming material.
When the thin film forming material used in the method of the present invention is oligophenylene or a derivative thereof, it is preferable, for example, to form, on the substrate, a self-assembly film made up of molecules each having a benzene ring at a terminal end or in its part. Also, when the thin film forming material used in the method of the present invention is oligothiophene or a derivative thereof, it is preferable, for example, to form, on the substrate, a self-assembly film made up of molecules each having a thiophene ring at a terminal end.
Further, the aforesaid feature a contains a method including: forming, on the substrate, a first area in a surface condition having a high affinity with the thin film forming material (i.e., an area corresponding to a thin film formed area) and a second area in a surface condition having a low affinity with the thin film forming material, and ejecting the solution such that the droplets are formed on the first area to entirely cover the first area.
In the above method, by way of example, the first area in the surface condition having a high affinity with the thin film forming material is disposed in a predetermined pattern, whereupon the remaining area on the substrate other than the first area defines the second area in the surface condition having a low affinity with the thin film forming material. Thereafter, the solution is ejected.
When a plurality of first areas are provided on the substrate, the solution is ejected such that the droplets are formed to entirely cover each of the plural first areas.
According to the above method, even when the thin film forming material dissolved in the ejected solution is formed with the droplets spreading over the second area upon ejection of the solution by the ink jet method, the droplets remain within the first area on the substrate, which is in the surface condition having a high affinity with the thin film forming material, and no droplets are present within the second area on the substrate, which is in the surface condition having a low affinity with the thin film forming material.
In the method of the present invention employing a low-molecular compound as the thin film forming material, therefore, a thin film is formed in the first area because the binding force between the thin film forming material and the substrate becomes higher than the cohesion among molecules of the thin film forming material. In the second area, however, any thin film forming material is not present and no particles of the thin film forming material are deposited. As a result, a crystalline thin film is formed on the substrate in the same pattern as the first area.
In the above method, preferably, the first area is provided by forming, on the substrate, a first self-assembly film made up of molecules each having an atomic group common to molecules constituting the thin film forming material, and the second area is provided by forming, on the substrate, a second self-assembly film made up of molecules each not having any atomic group common to molecules constituting the thin film forming material.
Further, the present invention provides a solution containing a thin film forming material and a solvent and used in a thin film forming method including: ejecting the solution and arranging droplets of the solution on a substrate, the solution being any of (1) a solution containing the thin film forming material in amount at which the solution is brought into the saturated state at the time of ejection of the solution, (2) a solution containing the thin film forming material in amount at which concentration of the solution is not less than {fraction (1/10)} of the saturated concentration but less than the saturated concentration at the time of ejection of the solution, (3) a solution containing the thin film forming material in amount at which the solution is brought into the supersaturated state at the time of ejection of the solution, (4) a solution in which a solvent is a poor solvent, (5) a solution prepared such that the saturated concentration of the thin film forming material with respect to the solvent is not less than 0.1 weight % but not more than 10.0 weight % at the time of ejection of the solution, and (6) a solution prepared such that the saturated concentration of the thin film forming material with respect to the solvent is not less than 0.1 weight % but not more than 1.0 weight % at the time of ejection of the solution.
Still further, the present invention provides a solution that satisfies any of the above conditions (1) to (6) and is given as one of: (7) a solution in which the solvent is dodecylbenzene, (8) a solution in which the solvent is 2,3-dihydrobenzofuran, (9) a solution in which the solvent is dimethylformamide, (10) a solution in which the thin film forming material is oligophenylene or a derivative thereof, (11) a solution in which the thin film forming material is oligothiophene or a derivative thereof, (12) a solution in which the thin film forming material is oligophenylene and the solvent is 2,3-dihydrobenzofuran, (13) a solution in which the thin film forming material is oligothiophene and the solvent is dodecylbenzene, and (14) a solution in which the thin film forming material is a derivative of oligophenylene or a derivative of oligothiophene and the solvent is dimethylformamide.
Still further, the present invention provides a thin film forming apparatus (first thin film forming apparatus) including: a stage on which a substrate is placed, an ejection device to eject, as droplets, a solution prepared by dissolving a thin film forming material in a solvent, and a gas component adjusting device to change a gas composition in a space above the stage.
Still further, the present invention provides a thin film forming apparatus (second thin film forming apparatus) including: a stage on which a substrate is placed, an ejection device to eject, as droplets, a solution prepared by dissolving a thin film forming material in a solvent, a gas introducing device to introduce a gas of a predetermined composition in a space above the stage, and a depressurizing device to reduce a pressure in the space above the stage.
According to the thin film forming method of the present invention, as described above, a very small amount of solution arranged in the predetermined position on the substrate, can be crystallized. As a result, a patterned crystalline thin film can be easily formed on the substrate by the ink jet method.
Also, according to the thin film forming apparatus of the present invention, the thin film forming method of the present invention can be easily implemented.
Moreover, as a first method for achieving the above more specific object, the present invention provides a thin film forming method including: ejecting a solution prepared by dissolving a thin film forming material in a solvent, arranging a plurality of droplets of the solution on a substrate, and evaporating the solvent from each droplet to form a thin film on the substrate. The method further includes: making control such that a gas made up of the same components as those of the solvent is present around the droplets.
With the above method, the partial pressure of the aforesaid gas around the droplets can be increased to such an extent as not causing a problem of unevenness in the partial pressure of the gas around the droplets, which arises for the droplet formed at the most end of a line as described above. In other words, the partial pressure of the gas around the droplets can be kept uniform at a high value. As a result, it is possible to reduce or prevent movement of even the aforesaid droplet that is formed at the most end of a line.
The above control step can be implemented by a manner of coating the solvent over the substrate.
One practical manner includes, for example, coating the solvent over the substrate, and then ejecting the solution on a section over which with the solvent has been coated.
In the method of the present invention, the solvent may be coated over the entire substrate including positions in which the droplets of the solution are to be arranged, or may be coated only in positions in which the droplets of the solution are not to be arranged. From the viewpoint of arranging the droplets of the solution on the substrate which is in a condition perfectly free from adverse effects, such as contamination caused by the arranged solvent, however, the solvent is preferably coated only in positions in which the droplets of the solution are not to be arranged.
In the method of the present invention, the solvent can be coated by any of various methods such as spin coating, ink jetting, dipping, and spray coating. When coating the solvent over the entire substrate, the spin coating method is preferable from the viewpoint of workability.
When coating the solvent by the ink jet method, droplets of the solvent may be arranged before arranging the droplets of the solution, or at the same time as arranging the droplets of the solution, or after arranging the droplets of the solution. When arranging the droplets of the solvent after arranging the droplets of the solution, the droplets of the solvent must be arranged immediately (e.g., within 1 second) after arranging the droplets of the solution.
Also, when coating the solvent by the ink jet method, the droplets of the solvent and the droplets of the solution are preferably formed such that the droplets of the solvent are evenly arranged around the droplets of the solution on the substrate.
As a second method for achieving the above more specific advantage, the present invention provides a thin film forming method including: ejecting a solution prepared by dissolving a thin film forming material in a solvent, arranging a plurality of droplets of the solution on a substrate, and evaporating the solvent from each droplet to form a thin film on the substrate. A low vapor-pressure solvent having a vapor pressure of not higher than 1.3xc3x9710xe2x88x923 Pa (1xc3x9710xe2x88x925 mmHg) at the time of ejection of the solution is used as the solvent.
With the above method, since the solvent is hard to evaporate from each of the droplets, each droplet is less susceptible to the effect of solvent vapors evaporated from the adjacent droplets even when the adjacent droplets are formed in positions relatively close to each other. As a result, even the aforesaid droplet formed at the most end of a line can be surely prevented from moving.
As a third method for achieving the above more specific advantage, the present invention provides a thin film forming method including: ejecting a solution prepared by dissolving a thin film forming material in a solvent, arranging a plurality of droplets of the solution on the substrate, and evaporating the solvent from each droplet to form a thin film on the substrate. The method further includes: after forming a first droplet, forming a second droplet in a position away a predetermined distance (distance sufficient to make negligible the effect of solvent vapors evaporated from the first droplet) from a position in which the first droplet has been formed.
With the third method, by forming the second droplet in the position in which it is not affected by solvent vapors evaporated from the first droplet, even the aforesaid droplet formed at the most end of a line can be surely prevented from moving.
Also, when the interval between adjacent two of the formed droplets is shorter than the predetermined distance (distance sufficient to make negligible the effect of solvent vapors evaporated from the first droplet), such a situation can be coped with the third method by: after forming the first droplet in a certain position, forming the next second droplet in a position away the predetermined distance from the position in which the first droplet has been formed, instead of a position in which the adjacent droplet is to be formed, and forming a droplet in the position, in which the adjacent droplet is to be formed, after drying of the first droplet.
As a fourth method for achieving the above more specific advantage, the present invention provides a thin film forming method including: ejecting a solution prepared by dissolving a thin film forming material in a solvent, arranging a plurality of droplets of the solution on the substrate, and evaporating the solvent from each droplet to form a thin film on the substrate. The method further includes: forming a second droplet in a position adjacent to a first droplet after drying of the first droplet.
With the fourth method, since the later-formed droplet (second droplet) is formed after drying of the droplet (first droplet) having been formed in the adjacent position, each droplet is less susceptible to the effect of solvent vapors evaporated from the adjacent droplet. As a result, even the aforesaid droplet formed at the most end of a line can be surely prevented from moving.
The thin film forming material usable in the methods described above is, for example, oligophenylene or a derivative thereof, or oligothiophene or a derivative thereof.
A crystalline thin film can be formed by the ink jet method by, after carrying out any of the above first to fourth methods, creating crystalline nuclei in the droplets by bringing the solution forming the arranged droplets into a supersaturated state and controlling a partial pressure of a gas made up of the same components as those of the solvent in the vicinity of the droplets to a first partial pressure under which the solvent is hard to evaporate from the solution forming the droplets, and after creation of the crystalline nuclei, lowering the partial pressure of the gas in the vicinity of droplets to a second partial pressure under which growth of the crystalline nuclei into crystals occurs with priority to creation of further crystalline nuclei.
According to the (first to fourth) thin film forming methods of the present invention, as described above, a thin film of a low-molecular compound can be formed in a predetermined position on a substrate. Also, by applying any of those methods to the method of forming a patterned crystalline thin film by the ink jet method, the crystalline thin film can be more easily formed in the predetermined position on the substrate with stability corresponding to a pattern. It is hence easy to form the patterned crystalline thin film.
In addition, the present invention provides an electronic device fabricating method including: forming a thin film in accordance with the thin film forming method of the present invention.